Overview

Distribution

Proboscis bats (Rhynchonycteris naso) can be found from Veracruz and Oaxaca in southeastern Mexico, down through Central America and reaching the northern half of South America including southeastern Brazil, Peru and northern Bolivia.

Biogeographic Regions: neotropical (Native )

Goodwin, G. 1942. Mammals of Honduras. Bulletin of the American Museum of Natural History, 79: 107-195.

Hall, E. 1981. The Mammals of North America. New York: John Wiley & Sons.

Koopman, K. 1982. Biogeography of the bats of South America. Pp. 273-300 in M Mares, H Genoways, eds. Mammalian Biology of South America, Vol. 6. University of Pittsburgh: Pymatuning Laboratory of Ecology.

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Range Description

This species ranges from Oaxaca and Veracruz, Mexico, to central and eastern Brazil, Peru, Bolivia, Guianas, Surinam and Trinidad (Simmons 2005). It is widely distributed at low elevations, generally below 500 m (Eisenberg 1989) but up to 1,500 m.

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Physical Description

Morphology

Proboscis bats are very small members of family Emballonuridae with gray-grizzled brown dorsal sides and two faint white stripes forming a distinct hourglass shape on their lower back and rump. Their ventral sides are pale brownish gray and their pelage is soft and dense, with a dark brown patagium. Their dark brown ears are prominent and tapered distally, much like their tragus. Young bats are slightly darker in color than adults. There is no significant sexual dimorphism, males and females are similar in appearance. Their body mass ranges from 3.8 to 3.9 g, their body length ranges from 37 to 43 mm including their tail, which has an individual length of 15.4 to 16.8 mm. When pregnant, females can weigh up to 6 g. Their average wingspan is 23.9 mm, and their wing aspect ratio (6.54) and wing loading (0.045) are considered intermediate when compared with 25 Neotropical species representing several families. The baculum of male proboscis bats is considerably larger than that of 6 other emballonurid species. Their dental formula is i 1/3, c 1/1, p 2/2, m 3/3 = 32 teeth. Their upper incisors are minute and distinctly separated, while the first upper premolar is relatively large, somewhat triangular in occlusal view and closer to the canine than to the last premolar. Skull lengths range from 11.4 to 12 mm in males and 11.6 to 12 mm in females. Some distinct features of this species include a deep basisphenoid pit and nearly parallel maxillary toothrows. The postorbital process and auditory bullae are well-developed. Similar and closely related emballonurids with overlapping ranges include greater sac-winged bats (Saccopteryx bilineata) and lesser sac-winged bats (Saccopteryx leptura). Proboscis bats can be distinguished from these species by their elongated muzzle and evenly-spaced tufts of white to pale gray fur along their forearms. The dorsal side of greater and lesser sac-winged bats is a darker shade of brown and lacks the gray-grizzled pattern found in proboscis bats. Male proboscis bats also lack wing sacs, which are organs on the propatagial membrane of the wings that store secretions used for mating rituals and are very prominent in greater and lesser sac-winged bats.

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Ecology

Habitat

Proboscis bats prefer tropical lowlands with an elevation of less than 300 m and are almost always found near or over moving water, but rarely near fast-moving water. Unlike most nocturnal bats, light does not seem to disturb the colony, individuals roost in well-lit areas, usually around 1.8 meters (6 feet) above water. These bats are known to cling upside down in a vertical line on the bark and roots of trees overhanging water, but on occasion, some have been found under bridges, in cave mouths overhanging water, under large curled leaves, such as those of banana plants and under large fabric umbrellas of outdoor Brazilian restaurants.

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Habitat and Ecology

It is almost always associated with moist areas near multistratal evergreen forests. These bats tend to roost in small, single-species colonies of about ten to twenty-four, on tree trunks, in tree cavities, or in rock caves (Eisenberg 1989). When roosting they are often aligned in vertical rows with individuals about 10 mm apart. Several males occur in a roosting group, and there appears to be no harem formation or defense. These bats are aerial insectivores (Husson 1978; Goodwin and Greenhall 1961); and they tend to feed over water, flying only a short distance above the surface (Eisenberg 1989). In Mexico they have been also reported for secondary forests, crop-lands and grasslands (de Grammont pers. comm.)

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Trophic Strategy

The diet of proboscis bats consists exclusively of insects, mainly those from order Diptera, such as midges and mosquitoes, although some beetles (order Coleoptera) and caddis flies (order Trichoptera) have also been found in stomach content analyses. Since Diptera, Coleoptera, and Trichoptera are thought to comprise approximately 87 to 90% of aerial insects found over water, a study has suggested that this species feeds on the most abundant insects in its foraging area.

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Associations

There has been no specific study on the role this species plays on the ecosystem, but due to its diet and large population numbers, proboscis bats may contribute to the control of insect populations in its habitat. In Panama, where these bats occur, research has shown that insectivorous bats indirectly affect herbivory on plants, by reducing herbivorous arthropod abundance. Proboscis bats are hosts to two known internal parasites: a coccidian parasite, Eimeria rhynchonycteridis, and a trypanosome, Trypanosoma cruzi. Some ectoparasites previously found on these bats include polyctenid hemipterans, Hesperoctenes fumarius, three species of streblid batflies: Strebla hirsutus, Trichobius caecus, and T. longipes; and two species of acarine mites: Eutrombicula variabilis and Periglischrus iheringi. However, the precise effects of these parasites have yet to be documented.

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Common predators of proboscis bats include various species of hawks, falcons, and egrets, although there have been documented cases of predation by orb-weaving spiders and northern annulated tree boas. This species can be very hard to detect when motionless, due to its pelage pattern, enabling it to remain cryptic to avoid predators. It also exhibits a cryptic behavioral adaptation where periods of synchronous gentle rocking can be observed throughout the day by most, if not all members of the colony in the absence of any threat. This behavior may also be more likely to occur during gusts of wind and with the combination of synchronous grooming and urination. Since this species tends to roost in open areas and is visually exposed, this behavior may benefit the colony by hiding its movements from potential predators.

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Life History and Behavior

Behavior

As nocturnal aerial insectivores, proboscis bats rely heavily on echolocation for prey detection. Their calls include a combination of high intensity signals, with narrowband (constant frequency) and broadband (frequency-modulated) components, of which, the former postulates details about small insect prey, while the latter provides information regarding the bat’s position relative to its surrounding. Proboscis bats emit high frequency calls of approximately 100 kHz, as well as lower frequencies around 47 kHz. High frequency calls indicate short-range detection of small prey in cluttered habitats, while lower frequencies increase the detection range, although only larger prey are perceived. Most calls are short, to prevent the overlap of outgoing and returning echoes from obstacles and/or prey. Complex social signals have not been observed in this species, although vocalizations by both sexes were observed when interacting with intra- and inter-colony newcomers.

Communication Channels: acoustic

Perception Channels: tactile ; echolocation ; chemical

Fenton, M., J. Rydell, M. Vonhof, J. Eklof, W. Lancaster. 1999. Constant-frequency and frequency-modulated components in the echolocation calls of three species of small bats (Emballonuridae, Thyropteridae, and Vespertilionidae). Canadian Journal of Zoology, 77: 1891-1900.

O'Farrell, M., B. Miller. 1997. A New Examination of Echolocation Calls of Some Neotropical Bats (Emballonuridae and Mormoopidae). Journal of Mammalogy, 78: 954-963.

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Life Expectancy

The calculated annual survival rate of adult females is at least 79%, but no lifespan record of this species has been found. However, their close relative, greater sac-winged bats (Saccopteryx bilineata) can live up to 6 years in the wild. A study of the factors affecting longevity showed that lifespan tends to increase with body mass, hibernation and cave use, but is negatively impacted by increased reproductive rates. Out of 64 species of bats, Saccopteryx bilineata is the most similar to proboscis bats, neither of these bats hibernate or roost in caves frequently; however, they both have a similar body mass and a similar number of offspring annually.

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Reproduction

Proboscis bats are polygynous and although the male to female ratio is usually equal, breeding females mainly copulate with dominant males. The rest of the colony likely follows a hierarchy, where reproductive females rank higher than non-reproductive females and non-dominant males. Dominant males forage at the edge of the feeding area and protect their colony from neighboring conspecifics using aerial attacks and audible vocalizations. Dominant males are thought to exhibit female-defense over resource-defense, since the males follow the females as they forage, even when they return to their roosting sites at night. Mating in proboscis bats is not well-studied, but during an observed copulation, two males approached a female from opposite sides, leading to what appeared to be inaudible vocalizations by the female. One male proceeded to edge over the female’s body and appeared to vocalize inaudibly, followed by copulation. The female and the second male then took off and executed a downward spiraling flight, until nearly reaching the water surface. However, the purpose of this act is not yet known.

Mating System: polygynous

Throughout the year, breeding proboscis bats can be found in different parts of their distribution, but may cease breeding during the dry season, which is November to March in Costa Rica and April to September in southeastern Brazil. Females produce only one offspring per pregnancy, but may have up to two pregnancies annually and can therefore be polyestrus. Offspring are quite large at birth and can reach adult size within 2 weeks. Weaning occurs after 2 to 4 months, after which, the young bats disperse to nearby colonies. Females undergo their first parturition around 18 months of age. Overlap between lactation with the first young and gestation of a second offspring has been documented.

Breeding interval: Proboscis bats breed once or twice annually.

Breeding season: The breeding season continues throughout the year, except the dry season in Costa Rica and southeastern Brazil

Within one week of birth, young bats begin to venture away from their mother, but do not stray far. While the mothers are out foraging, young bats practice flying at the roost. Once the young are able to fly, they forage along with the breeding females in the central feeding area, until dispersal at 2 to 4 months old. Usually, females with offspring roost with the main colony, but some have been found in hollow logs, which are thought to minimize the risk of young bats falling into water. Aside from the general protection of the colony offered by the dominant male, there has been no record of paternal care in this species.

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This species is listed as Least Concern because, although it is widely distributed, common in areas with water and suitable habitat and unlikely to be declining at a rate which would qualify the species for inclusion in one of the threat categories in the near future.

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Trends

Population

These bats are often common in lowland forest near water (streams, rivers, mangroves, and lakes) (Reid 1997); widespread (Emmons and Feer 1997). Colonies vary from a few individuals to 100 individuals (Dalquest 1957).

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Management

Conservation Actions

This species occurs in protected areas. It is widely distributed throughout the Neotropics. In Mexico it is listed as subject to special protection under NOM - 059 - SEMARNAT - 2001 (Arroyo-Cabrales pers. comm.).

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Relevance to Humans and Ecosystems

Benefits

Proboscis bats can be infected with Trypanosoma cruzi, the parasite that causes Chagas Disease in humans. Although this parasite cannot be directly transmitted from bats to humans without incubation within an intermediate host, the bats can serve as a reservoir host for the development of more parasites.

Contents

This is a small bat, around 6 centimetres (2.4 inches) long and 4 grams (0.0088 pounds) in weight. Males in northern South America were found to average 56.48 millimeters long, females 59.18.[4] The tail is about 1.6 centimeters long.[4] Pregnant females can weigh up to 6 grams.[4] The species is characterized by its long, fleshy, and pointed nose. Its fur is soft and dense and is brownish-grey in color, with two white stripes down the back. Whether these stripes serve a purpose, such as camouflage or attraction of mates, is unknown. This bat also has gray tufts of fur on the forearms. No matter what time of day these features may make the bat difficult to see.

This species is found in the lowlands of the northern half of South America, throughout Central America, and into southeastern Mexico. From Ecuador south, it is limited to east of the Andes; its range extends south to the northern half of Bolivia and much of Brazil. It seldom occurs above 300 meters (980 feet) in elevation.[4] It usually lives around wetlands and is frequently found in riparian forests, pastures swamps, and all near water.

Proboscis bats live in groups. The colonies are usually between five and ten individuals, and very rarely exceed forty. The bats are nocturnal, sleeping during the day in an unusual formation: they lay one after another on a branch or wooden beam, nose to tail, in a straight row.

A colony of proboscis bats usually has a regular feeding area, typically a small patch of water. Here the bats catch insects using echolocation. They have no specific breeding season, forming stable year-round harems. One young is born. Both sexes disperse after weaning at around 2–4 months.

This small species of bat has been found to occasionally fall prey to the large spider Argiope savignyi.[7]